The bulk polymerization of ethylene derivatives, and notably of vinyl chloride, presents a number of difficulties due largely to the fact that the reaction is exothermic and therefore it is necessary to provide effective means for the correct and regular removal of the heat released during the reaction to ensure uniform properties in the products obtained.
During bulk polymerization, it is found that the whole of the material undergoing polymerization very rapidly adopts a pulverulent form. For example, in the case of vinyl chloride, this pulverulent form is reached as soon as a mere 20% of the material is polymerized. This pulverulent form does not favor heat exchange and the correct and regular removal of the heat released during the reaction through the walls of the autoclave, which is generally used as the reaction vessel.
Stirring this pulverulent material, so as to bring it into contact with the walls of the autoclave, enables the heat of reaction to be removed with some degree of success. It is, however, difficult, with the current stirring or agitation means, to obtain a homogeneous dispersion of the solid particles of polymer in the vapor phase of the monomer. This irregular distribution of the polymer particles during growth, resulting from the polymerization of the adsorbed monomer which the particles contain, leads to the formation of aggregates which, at the end of the polymerization, frequently result in the polymer of a single batch having widely varying grain size, and may even result in crust formation.
Stirrers of the "ribbon blender" type have already been used. They comprise one or more helically wound strips fixed coaxially to a single rotatable shaft passing axially through the autoclave along its axis. This type of stirrer is based on the principle of a trough mixer in which two helically wound strips of "balanced opposed pitch" rotate. These two strips operate in the manner of a pump, one of the strips moving the material in one direction and the other in the opposite direction. When the pitches are balanced, there is no resultant transport of the material to either one end or the other of the trough. This is acceptable when the trough is filled and when the volume of material does not vary during the course of stirring or mixing. During polymerization, however, the density of the material varies and the portion of the stirrer which is immersed varies and it is found that transportation and accumulation of the material at one of the ends takes place, with the result that the stirring efficiency is reduced.
In my French Pat. No. 1,360,251, a stirrer of a frame type is proposed for overcoming this disadvantage. This stirrer comprises one or more frames fixed coaxially to a single rotatable shaft passing axially through the autoclave, the periphery of the frame, forming a blade which, when the stirrer is rotating, moves in close proximity to the internal surface of the autoclave.
Nevertheless, when using stirrers of the "ribbon blender" or "frame" type, formation of crusts on the surface of the stirrer shaft is observed during polymerization. The shaft of the stirrer also has the following disadvantages:
It contributes, to only a slight extent, to the inertia and therefore the rigidity of the stirrer, while considerably increasing its volume. The rigidity of a stirrer does in fact increase with its inertia, and to obtain a given inertia, more and more material is required, the closer this material is situated to the axis of rotation.
It does not, in practice, contribute at all to the stirring or agitation of the reaction medium.
It makes only a small contribution to heat exchange between the stirrer and the reaction medium for two reasons; firstly, for a given flow rate of heat exchange fluid within the stirrer, the speed of flow of fluid inside the shaft is relatively slow; secondly and chiefly, because of the slow movement of the reaction medium in the vicinity of the autoclave axis.
It prevents introduction of ingredients along the autoclave axis. The ingredients can be introduced only through an opening formed in the wall of the autoclave. The passage of the stirrer blades past the opening, on each revolution of the stirrer, has a disadvantageous effect upon the distribution of the ingredients and can lead to the formation of deposits.
It is an object of this invention to provide a means and method for bulk polymerization of monomers which produce polymers insoluble in the monomers in a manner which overcomes the heat exchange and temperature control problems previously described and provides for an improved production of polymers of more uniform particle size.